The planar integration method is more suitable for the development of practical application of metamaterial devices due to its convenient processing and ultra-thin design. Instead of multi-layer stacking, another alternative is to prepare a two-dimensional planar layout by integrating resonators of various resonant frequencies into one meta-tom . Meanwhile, it will be difficult to miniaturize and costly to manufacture. Nevertheless, the multilayer approach will bring new issues that the absorbers involve complex designs and fabrication become thick and bulky. For example, the EAB can be extended by means of laminated resonators and dielectrics ,.
Superimposing different resonant modes into one unit cell is the most straightforward approach.
Until now, considerable efforts have been devoted to broad the EM response bandwidth of metamaterials . Since a single band metasurface absorber is inappropriate for most microwave device applications, development of high-performance metasurface with multiband or wideband in compact dimensions is essential. Unfortunately, due to the natural resonance feature and the corresponding high quality factor, the effective absorption bandwidth (EAB, reflection loss value less than −10 dB) of the most metamaterial with MDM configuration is relatively narrow, akin to the traditional Salisbury Screen . The magnetic dipole can tune the permeability μ in turn, so that the intrinsic impedance of metamaterial Z = μ / ɛ matches the free-space impedance Z 0, leading to considerable sum of incident EM waves entering into the devices . The near-field coupling between the two metal layers forms resonant currents, which generates a magnetic dipole in the dielectric spacer. Since then, various types of metasurface (two-dimensional metamaterial) based on the metal–dielectric–metal (MDM) construction have been put forward and developed by taking advantage of light weight, flexible design, and high absorption . in 2008 , it rapidly inspired multitudinous works ranging from microwave and terahertz to visible regime as a result of the potential application prospects. Since the perfect metamaterial absorber was first proposed by Landy et al. Conventional MAMs such as carbon absorbing materials , ferrites magnetic materials , and conductive polymers are suffering from disadvantages of large thickness, high weight or complicated fabricating process, consequently restricting their practical applications in much more fields. Microwave absorbing materials (MAMs) are widely used in various areas ranging from sensing, electromagnetic (EM) shielding, radar stealth technology, to energy harvesting, for effectively eliminating or absorbing unwanted EM radiation or scattering in order to reduce EM pollution .
These results demonstrate that the synthetic fractal metasurface can be a good candidate absorber for microwave applications like sensing, multiband detecting and filtering. Experimental microwave average absorptivity over 82.9% (reflection loss, RL ≤−7 dB) covering the 10.82–14.18 GHz region is obtained and the maximum absorptivity exceeds 99.8% with sub-wavelength thickness (0.039 λ 0). Multiple absorption modes are achieved through the synergistic effect of multiple LC-resonances and cross-coupling of the patterned elements, and a broadband operation is completed by adjusting the thickness of dielectric layer based on interface interference theory. Herein, we demonstrate a microwave metasurface absorber consist of periodic supercells of Fibonacci spiral capable of achieving highly efficient absorptions in a certain bandwidth and several discrete frequencies. Synthetic fractals have a large potential to achieve tunability and broadband serviceability for multimodal microwave devices owing to their scale-invariant property that generate strongly enhanced local fields with multiscale spatial distributions over multi-spectral ranges.
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